The central medical-records linkage system at the Mayo Clinic for the population of Olmsted County, Minnesota, provided the necessary data resource for the identification and follow-up of virtually all diagnosed cases of the Guillain--Barré syndrome over a period of 42 years. During this time, a total of 40 Olmsted County residents had clinical signs and symptoms of the disease. The mean annual incidence rate was 1.7 per 100,000 population. There was a significantly higher incidence rate (P less than 0.01) among patients 40 years of age and older. In four patients malignancies developed; however, the occurrence of the Guillain-Barré syndrome had no significant effect on survivorship in these patients. A case-control comparison of suggested etiologic factors supports the view that there is a significant association between antecedent infections and the Guillain-Barré syndrome.